The present invention relates to the modification of an electromagnetic signal through interaction with an energy beam-induced moving plasma in a semiconductor. The modified signal is frequency up-shifted, producing a fast electromagnetic or electrical signal.
A prior art method of producing fast electromagnetic signals utilizes a photoconductive switch. A semiconductor is placed between two contacts which are connected to a voltage source in the external circuit. The semiconductor behaves like an insulator until it is made to conduct with laser illumination of the proper wavelength. A fast pulse of laser energy will thus produce a fast electrical signal in the external circuit. One basic version of this switch gives an electrical signal amplitude that is approximately proportional to the optical energy deposited in the semiconductor. The other basic version uses semiconductors such as GaAs which can turn on completely when the illumination energy is above some threshold. The basic mechanism for this latter version is referred to as optically initiated avalanche. Discussions of these types of switches can be found in the following U.S. Patents: Davis U.S. Pat. No. 4,438,331; Ragle U.S. Pat. No. 4,864,119; and Kim U.S. Pat. No. 5,028,971. The general advantage of photoconductive switches is their ability to produce fast, high amplitude pulses, for example, a 2.5 kV electrical pulse with a rise time of 200 picoseconds (ps). More generally, a laser illumination of the proper wavelength and energy between two separated conductors on a semiconductor substrate will electrically connect or "short" the two conductors. Thus, by turning on such illumination, it is possible to make an electrical connection for as long as the optical energy is applied. For a review of this technology see Lee, "Picosecond optics and microwave technology," IEEE Trans. Microwave Theory Tech., vol. MTT-38, pp. 596-607, 1990.
Another prior art method to produce fast microwave signals was recently reported by Savage et al. in "Frequency Upconversion of Electromagnetic Radiation upon Transmission into an Ionization Front," Physical Review Letters, vol. 68, Feb. 1992. This paper describes an experiment where an optically induced moving ionization front in a gaseous medium interacts with an impinging microwave signal, producing an up-shifted signal. Source radiation at 35 GHz was up-converted to 116 GHz when an ionizing laser pulse was propagated through a resonant microwave cavity. However, the tens of mJ of optical pulse energy used was inadequate to produce a true reflective plasma at microwave frequencies, giving up-shifts different than those predicted by a simple Doppler effect. The up-conversion under these conditions was rather inefficient, being less than 1% at 116 GHz.
An ideal situation for efficient frequency up-conversion would be to produce an optically induced moving ionization from which is sufficiently dense to give complete reflection for an impinging electromagnetic signal. Such a case is analogous to the reflection of electromagnetic radiation from a moving mirror, which will give a Doppler shift: dependent on the mirror's velocity. The up-conversion factor due to the relativistic Doppler effect in the rest frame of the observer for an ideal reflecting "front" moving toward the impinging electromagnetic radiation is given by equation 1 which is written and plotted in FIG. 1. The velocity of the electromagnetic radiation in the medium is "c" and the velocity of the moving reflecting front is "v". As the velocity of the reflecting boundary becomes a significant fraction of the velocity of the electromagnetic radiation, very large up-shifts will occur.
Because of the technological difficulty of producing fast laser pulses of sufficiently high energy to create a truly reflective ionization front in a gas, frequency up-conversion by the pure Doppler effect has not previously been achieved. The low efficiency up-conversion demonstrated by Savage et al. was due to a plasma-microwave interaction which is a superset of the Doppler effect.